Coded modulation using repetition and tree codes

ABSTRACT

A transmitter includes a repetition encoder generating a repetition code sequence having a word length of r bits by carrying out a repetition coding with respect to 1 bit of a transmission unit which divides transmission information by a predetermined word length, where a rate of the repetition coding is set equal to a reciprocal of a predetermined integer r, a tree encoder generating a tree code sequence by carrying out a tree coding with respect to the other bits of the transmission unit, where a rate of the tree coding is set to k/n, k denotes an information block length and n denotes a code block length, a mapping part carrying out a mapping with respect to a combination of the repetition code sequence generated by the repetition encoder and the tree code sequence generated by the tree encoder, based on a set partitioning while maintaining correspondence between the mapping and the transmission unit, and a modulator generating a transmitting wave signal by carrying out an M-ary modulation scheme with respect to a carrier wave signal based on the combination which is mapped by the mapping part, and transmitting the transmitting wave signal to a radio channel. The predetermined word length is (1+rk/N) bits, and N denotes a number of transmission symbols indicated by a single branch of a trellis diagram indicating a sequence of the tree code sequence. The modulator modulates the carrier wave signal by 2.sup.(1+n/N) levels.

BACKGROUND OF THE INVENTION

The present invention generally relates to transmitters and receivers,and more particularly to a transmitter which employs a coded modulationtechnique and to a receiver adapted to receive information from such atransmitter.

In digital radio communication systems, an M-ary modulation scheme is inmany cases applied to a radio transmission channel. Particularly in thecase of a mobile communication system, proposals have been made to applya coded modulation technique employing the M-ary modulation scheme inorder to simultaneously realize reduction in the power consumed at amobile station and effective utilization of finite radio frequencies.

FIG. 1 is a system block diagram showing an example of a transmitteremploying the coded modulation technique which is called trellis codedmodulation (TCM) or block coded modulation (BCM) scheme. In FIG. 1,transmission information is divided into n-bit blocks. 1 bit of eachn-bit block is input to an M-ary modulator 165, while the remaining n-1bits are input to the M-ary modulator 165 via an encoder 166. Atransmitting wave signal is obtained from an output of the M-arymodulator 165, and this transmitting wave signal is supplied to atransmitting part which is not shown.

In the transmitter having the construction shown in FIG. 1, the encoder166 generates a code sequence by subjecting the n-1 bits of each n-bitblock to a trellis coding (or block coding) at a rate R of (n-1)/n. TheM-ary modulator 165 employing (n+1)-bit level reads the n-bit codesequence and the 1 bit which is not encoded, and generates thetransmitting wave signal by subjecting a carrier wave signal to a(n+1)-level modulation based on the two.

For simplicity, if it is assumed that an 8-phase phase shift keying isemployed, signal points of the transmitting wave signal which isgenerated in the above described manner become as shown in FIG. 2 by theM-ary modulator 165 by mapping symbols a₁, a₂ and a₃ (n=2 in this case)based on the set partitioning. Accordingly, the symbols a₁, a₂ and a₃and the signal points are set to values which satisfy a correspondingrelationship C₁ Δ₁ =C₂ Δ₂ =C₃ Δ₃ when a uniform error protection ispossible, where C₁, C₂ and C₃ denote minimum distances of the codes foreach bit level, respectively, and Δ₁, Δ₂ and Δ₃ denote minimum distancesbetween the signal points in the signal space. In this case, assuming anon-coded bit is a₁, C₁ Δ₁ =C₂ Δ₂ is required ebcause of C₂ =C₃.

Furthermore, at a receiving end which receives and demodulates thetransmitting wave signal described above, the structure of the trellisdiagram becomes simple because the received signal to be demodulated isgiven by a single coding level, and a maximum likelihood decoding basedon Viterbi decoding can be carried out efficiently.

FIG. 3 is a system block diagram showing another example of thetransmitter employing the coded modulation, which is called amulti-level coded modulation (MLCM) scheme. In FIG. 3, the transmissioninformation is input to a serial-to-parallel converter 170, and Moutputs of the serial-to-parallel converter 170 are input to a mappingpart 172 via corresponding encoders 171₁ through 171_(M). An output ofthe mapping part 172 is input to a modulator 173, and the transmittingwave signal is obtained from an output of the modulator 173.

In the transmitter having the construction shown in FIG. 3, theserial-to-parallel converter 170 divides the transmission informationinto units of M bits and carries out a serial-to-parallel conversion.The encoders 171₁ through 171_(M) independently encode the M groupswhich are obtained in parallel from the serial-to-parallel converter 170at a desired coding level. The mapping part 172 and the modulator 173modulate a carrier wave signal depending on each of the block codeswhich are generated by the encoding, so as to generate the transmittingwave signal.

The corresponding relationship of the signal points of the generatedtransmitting wave signal and the symbols corresponding to the signalpoints is similar to that of the example shown in FIG. 1, and adescription thereof will be omitted.

According to the transmitter shown in FIG. 3, the encodings at theindividual bit levels are carried out in parallel, and the rate of theencoding can positively be set to a desired value. For this reason, itis possible to secure the minimum value of the distance between thesignal points of each individual group (bit level) to a higher value ascompared to the example shown in FIG. 1.

The asynchronous transfer mode (ATM) is a transmission system forrealizing mainly a broadband integrated services digital network(B-ISDN), and the precondition is to make the transmission by wire,particularly by optic fiber. Hence, it is a precondition that the biterror rate (BER) in a satisfactory state is 10⁻¹¹ or less.

In general, the required BER of the header is 10⁻⁷ to 10⁻¹¹ or lesssince the cell loss rate affects the system performance, and therequired BER of the data is 10⁻⁶ or less for images or the like.

On the other hand, the BER performance of the radio communication ispoor, and the radio communication is mainly used for voice transmissionwith a required BER of 10⁻² or greater and for low-speed data on theorder of several kbps. Furthermore, in the mobile communication systems,the BER performance is floored due to multipath fading.

In such a small channel capacity, it is not effective, bothfrequency-wise and power-wise, to make the overall BER performance to avery small value less than or equal to 10⁻¹¹, for example. For thisreason, in order to realize the ATM in the radio communication, it isnecessary to prepare 2 different channels for the header and the data,respectively, so that the required BER for the header is 10⁻⁷ to 10⁻¹¹or less and the required BER for the data is 10⁻⁶ or less. The BERperformance for the header is set very small because control informationsuch as destination information is included in the header, and the cellcannot be received if the contents or sequence of the headers aredamaged or changed due to the error. The cell which cannot be receivedmust be discarded, and then the system performance is heavily degraded.

On the other hand, in order to make a high-speed data transmission in achannel having a poor BER performance, it is essential to employ anerror correction technique including coded modulation schemes, and suchan error correction technique has come into practical use in satellitecommunications and some mobile communications. In addition, astechniques for compensating for the fading, it is known that a diversitytechnique, an adaptive antenna technique using directional antenna, andequalization are effective in eliminating the BER floor.

However, with respect to the conventional trellis coded modulation,block coded modulation, and multi-level coded modulation using thenon-coded bit level, a large number of bit errors are generated for thenon-coded bit level due to the fluctuation of the transmissioncharacteristic of the radio transmission channel, such as fading, andthe overall transmission performance gets a few gain.

On the other hand, according to the multi-level coded modulation notusing the non-coded bit level, or using more than one coding level, thetrellis structure becomes more complicated as the number of codinglevels increases and for this reason, a multi-stage decoding is carriedout at the receiving end. In other words, decoders D₁ through D_(M)sequentially carry out the decoding process under timings determined bythe multiple stages of delays provided by delay elements T₁ throughT_(M), and results of the decoded process are subjected to aparallel-to-serial conversion in a parallel-to-serial converter PSC asshown in FIG. 4. According to this multi-stage decoding, a decodingdelay equal to a sum of the delays provided by the delay elements T₁through T_(M) occurs, thereby degrading the real-time transmissionperformance. Furthermore, since the coding level in the higher layercannot use the decoded result of the coding level in the lower layer, amaximum likelihood decoding cannot be achieved and the performance isdegraded.

In addition, in the ATM network by use of the cells, the transmissionmust wait until a predetermined amount of information is filled in thecells. Hence, in such an ATM network, a transmission delay occurs whentransmitting low-speed data such as voice data because it takes time forthe information to be filled in the cells. On the other hand, whentransmitting high-speed data such as image data, a transmission delaysimilarly occurs in the process of carrying out error correction andre-transmission (ARQ: automatic repeat request) in order to meet arequired BER.

Furthermore, in order to efficiently realize the radio communicationemploying the ATM, it is conceivable to prepare 2 differentcommunication systems with different BERs, that is, one for the headerwith a BER of 10⁻⁷ to 10⁻¹¹ or less and another for the data with a BERof 10⁻⁶ or less. However, the efficiency becomes poor if 2 physicallydifferent channels are prepared, individually.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful transmitter and receiver in which the problemsdescribed above are eliminated.

Another and more specific object of the present invention is to providea transmitter and a receiver which can simplify the encoding process andthe decoding process, and can maintain a high BER and a high frequencyutilization efficiency particularly under fading environments. In otherwords, the goal of the present invention is to optimize the BERperformance over a fading channel, with a high efficiency and reasonabledelay.

Still another object of the present invention is to provide atransmitter comprising a repetition encoder generating a repetition codesequence having a word length of r bits by carrying out a repetitioncoding with respect to 1 bit of a transmission unit which dividestransmission information by a predetermined word length, where a rate ofthe repetition coding is set equal to a reciprocal of a predeterminedinteger r, a tree encoder generating a tree code sequence by carryingout a tree coding with respect to the other bits of the transmissionunit, where a rate of the tree coding is set to k/n, k denotes aninformation block length and n denotes a code block length, a mappingpart carrying out a mapping with respect to a combination of therepetition code sequence generated by the repetition encoder and thetree code sequence generated by the tree encoder, based on a setpartitioning while maintaining correspondence between the mapping andthe transmission unit, and a modulator generating a transmitting wavesignal by carrying out an M-ary modulation scheme with respect to acarrier wave signal based on the combination which is mapped by themapping part, and transmitting the transmitting wave signal to a radiochannel, where the predetermined word length is (1+rk/N) bits, N denotesa number of transmission symbols indicated by a single branch of atrellis diagram indicating a sequence of the tree code sequence, and themodulator modulates the carrier wave signal by 2.sup.(1+n/N) levels.According to the transmitter of the present invention, compared to theconventional trellis coded modulation, the block coded modulation andthe multi-level coded modulation using the non-coded bit level, the BERperformance over the fading channel is greatly improved at the receivingend. Further, compared to the conventional multi-level coded modulation,the decoding delay is compressed, and a maximum likelihood decoding canbe carried out efficiently based on a simple processing procedure.

A further object of the present invention is to provide the transmitterdescribed above, wherein the transmission unit is given by a formatwhich is made up of a combination of 1 bit corresponding to an order oftolerable upper limit values of a required BER for the repetition codeand the other bits, and the transmitter further comprises a dividerdividing the transmission unit into the 1 bit and the other bits basedon the format, and supplying the 1 bit to the repetition encoder theother bits to the tree encoder. According to the transmitter of thepresent invention, each of the bits forming the transmission unit aresubjected to the repetition coding or the tree coding respectivelycorresponding to a desired coding level. Hence, as long as thecombination of the number of these bits and the coding levels match, thebit error can be reduced and the maximum likelihood decoding can becarried out efficiently based on a simple processing procedure at thereceiving end.

Another object of the present invention is to provide a transmittercomprising a divider reading transmission units which are given by aformat in which transmission information is divided by a word having abit length A+B which is equal to a sum of A bits corresponding to anorder of tolerable upper limit values of a BER and B bits, andgenerating a first bit sequence and a second bit sequence by isolatingthe A bits and the B bits from the transmission unit based on the formatand adding first dummy bits and second dummy bits to the A bits and theB bits, respectively, a repetition encoder generating a repetition codesequence by extracting 1 bit at a time from the first bit sequencegenerated by the divider and carrying out a repetition coding at a rateequal to a reciprocal of a predetermined integer r, a tree encodergenerating a tree code sequence by extracting a predetermined number ofbits at a time from the second bit sequence generated by the divider andcarrying out a tree coding at a rate k/n, where k denotes an informationblock length and n denotes a code block length, a mapping part carryingout a mapping with respect to a combination of the repetition codesequence generated by the repetition encoder and the tree code sequencegenerated by the tree encoder, based on a set partitioning whilemaintaining correspondence between the mapping and the transmissionunit, and a modulator generating a transmitting wave signal by carryingout an M-ary modulation scheme with respect to a carrier wave signalbased on the combination which is mapped by the mapping part, andtransmitting the transmitting wave signal to a radio channel, wherein,with respect to a number N of transmission symbols indicated by a singlebranch of a trellis diagram indicating a sequence of the tree codesequence, a number of the first dummy bits for the first bit sequence is{max(A, BN/rk!)-A} and a number of the dummy second bits for the secondbit sequence is {max(A, BN/rk!)-BN/rk}·rk/N, the predetermined number isrk/N, and the modulator modulates the carrier wave signal by2.sup.(1+n/N) levels. According to the transmitter of the presentinvention, the word length of the transmission unit is corrected to avalue adapted to each desired coding level, and the coded modulation isapplied with the combination of the repetition coding and the treecoding. For this reason, the transmission information having variousformats can be transmitted by carrying out the coded modulation, and atthe receiving end, it is possible to reduce the bit error and toefficiently carry out the maximum likelihood decoding based on a simpleprocessing procedure.

Still another object of the present invention is to provide a receivercomprising a demodulator obtaining a reception sequence by demodulatinga reception wave received from a transmitting end via a radio channel, adecoder generating a decoded symbol sequence by carrying out a maximumlikelihood decoding with respect to the reception sequence obtained bythe demodulator, and a demapping part reading the decoded symbolsequence generated by the decoder, and restoring a transmission unit ofthe reception wave by carrying out a demapping adapted to a format of aset partitioning carried out by a mapping at the transmitting end whichtransmits the reception wave based on transmission information.According to the receiver of the present invention, it is possible topositively restore the transmission information (transmission unit)indicated by the reception wave which is received from the transmittervia the radio channel.

A further object of the present invention is to provide a receivercomprising a demodulator obtaining a reception sequence by demodulatinga reception wave received from a transmitting end via a radio channel, adecoder generating a decoded symbol sequence by carrying out a maximumlikelihood decoding with respect to the reception sequence obtained bythe demodulator, a demapping part reading the decoded symbol sequencegenerated by the decoder, and restoring a first bit sequence and asecond bit sequence which are generated by a demapping which is adaptedto a format of a set partitioning carried out by a mapping carried outat the transmitting end which transmits the reception wave based ontransmission information, and a word length corrector reading the firstbit sequence and the second bit sequence which are obtained by thedemapping part, and restoring a transmission unit by eliminating dummybits which are added to the first bit sequence and the second bitsequence by the set partitioning carried out at the transmitting end.According to the receiver of the present invention, it is possible topositively restore the transmission information (transmission unit)indicated by the reception wave which is received from the transmittervia the radio channel.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram showing an example of a transmitteremploying the coded modulation technique;

FIG. 2 is a diagram for explaining distribution of signal points basedon the set partitioning;

FIG. 3 is a system block diagram showing another example of thetransmitter employing the coded modulation;

FIG. 4 is a diagram for explaining a processing delay generated in theexample of the transmitter employing the multi-level coded modulationnot using the non-coded bit level;

FIG. 5 is a system block diagram for explaining the operating principleof a transmitter according to a first aspect of the present invention;

FIG. 6 is a system block diagram for explaining the operating principleof a receiver according to the first aspect of the present invention;

FIG. 7 is a system block diagram for explaining the operating principleof a transmitter according to a second aspect of the present invention;

FIG. 8 is a system block diagram for explaining the operating principleof a receiver according to the second aspect of the present invention;

FIG. 9 is a system block diagram showing a first embodiment of atransmitter and a receiver according to the present invention;

FIG. 10 is a diagram showing a trellis diagram of a code sequencegenerated by the embodiment;

FIG. 11 is a diagram showing a BER obtained by the embodiment;

FIG. 12 is a system block diagram showing a second embodiment of thetransmitter and the receiver according to the present invention; and

FIG. 13 is a system block diagram showing a third embodiment of thetransmitter and the receiver according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a description will be given of the operating principle of atransmitter according to a first aspect of the present invention, byreferring to FIG. 5.

A transmitter includes a repetition encoder 11, a tree encoder 12, amapping part 13 and a modulator 14 which are connected as shown in FIG.5. The repetition encoder 11 generates a repetition code sequence havinga word length of r bits by carrying out a repetition coding with respectto 1 bit of a transmission unit which is obtained by dividingtransmission information by a predetermined word length, and a rate ofthe repetition coding is set equal to a reciprocal of a predeterminedinteger r. On the other hand, the tree encoder 12 generates a tree codesequence by carrying out a tree coding with respect to the other bits ofthe transmission unit, and a rate of the tree coding is set to k/n,where k denotes an information block length and n denotes a code blocklength.

The mapping part 13 carries out a mapping with respect to a combinationof the repetition code sequence generated by the repetition encoder 11and the tree code sequence generated by the tree encoder 12, based onthe set partitioning while maintaining correspondence between themapping and the transmission unit. The modulator 14 generates atransmitting wave signal by carrying out an M-ary modulation scheme withrespect to a carrier wave signal based on the combination which ismapped by the mapping part 13, and transmits the transmitting wavesignal to a radio channel.

The predetermined word length is (1+rk/N) bits, where N denotes a numberof transmission symbols indicated by a single branch of a trellisdiagram indicating a sequence of the tree code sequence. In addition,the modulator 14 modulates the carrier wave signal by 2.sup.(1+n/N)levels.

According to this transmitter shown in FIG. 5, all bits forming thetransmission information are encoded and are subjected to codedmodulation and transmitted. In addition, the trellis diagram indicatingthe code sequence given by the above described mapping is given as acombination of 2 identical sub-trellis diagrams corresponding to theindividual logic values assumed by the repetition code sequence.

Therefore, compared to the conventional trellis coded modulation, theblock coded modulation and the multi-level coded modulation using thenon-coded bit level, the BER performance over the fading channel isgreatly improved at the receiving end. Further, compared to theconventional multi-level coded modulation employing more than one codinglevel, the decoding delay is compressed, and a maximum likelihooddecoding can be carried out efficiently based on a simple processingprocedure.

The transmission unit may be given by a format which is made up of acombination of 1 bit, and the other bits corresponding to the order oftolerable upper limit values of the required BER for the repetitioncode. In this case, a divider 21 indicated by a phantom line in FIG. 5is provided to divide the transmission unit into the 1 bit and the otherbits based on the format. The divider 21 supplies the 1 bit to therepetition encoder 11 and supplies the other bits to the tree encoder12.

In this case, each of the bits forming the transmission unit aresubjected to the repetition coding or the tree coding respectivelycorresponding to a desired coding level. Hence, as long as thecombination of the number of these bits and the coding levels match, thebit error can be reduced and the maximum likelihood decoding can becarried out efficiently based on a simple processing procedure at thereceiving end.

Next, a description will be given of the operating principle of areceiver according to the first aspect of the present invention, byreferring to FIG. 6.

A receiver includes a demodulator 41, a decoder 42 and a demapping part43 which are connected as shown in FIG. 6. The demodulator 41 obtains areception sequence by demodulating a reception wave received from thetransmitter shown in FIG. 5 via the radio channel. The decoder 42generates a decoded symbol sequence by carrying out a maximum likelihooddecoding with respect to the reception sequence obtained by thedemodulator 41. The demapping part 43 reads the decoded symbol sequencegenerated by the decoder 42, and restores the transmission unit bycarrying out a demapping adapted to the format of the set partitioningcarried out by the mapping part 13 of the transmitter shown in FIG. 5.

According to this receiver shown in FIG. 6, it is possible to positivelyrestore the transmission information (transmission unit) indicated bythe reception wave which is received from the transmitter via the radiochannel.

Next, a description will be given of the operating principle of atransmitter according to a second aspect of the present invention, byreferring to FIG. 7.

A transmitter includes a divider 31, a repetition encoder 32, a treeencoder 33, a mapping part 34 and a modulator 35 which are connected asshown in FIG. 7. The divider 31 reads transmission units which are givenby a format in which transmission information is divided by a wordhaving a bit length A+B which is equal to a sum of A bits correspondingto the order of tolerable upper limit values of the required BER and Bbits, and generates a first bit sequence and a second bit sequence byisolating the A bits and the B bits from the transmission unit based onthe format and adding first dummy bits for the first bit sequence andsecond dummy bits for the second bit sequence to the A bits and the Bbits, respectively.

The repetition encoder 32 generates a repetition code sequence byextracting 1 bit at a time from the first bit sequence generated by thedivider 31 and carrying out a repetition coding at a rate equal to areciprocal of a predetermined integer r. On the other hand, the treeencoder 33 generates a tree code sequence by extracting a predeterminednumber of bits at a time from the second bit sequence generated by thedivider 31 and carrying out a tree coding at a rate k/n, where k denotesan information block length and n denotes a code block length.

The mapping part 34 carries out a mapping with respect to a combinationof the repetition code sequence generated by the repetition encoder 32and the tree code sequence generated by the tree encoder 33, based onthe set partitioning while maintaining correspondence between themapping and the transmission unit. The modulator 35 generates atransmitting wave signal by carrying out an M-ary modulation scheme withrespect to a carrier wave signal based on the combination which ismapped by the mapping part 34, and transmits the transmitting wavesignal to a radio channel.

With respect to a number N of transmission symbols indicated by a singlebranch of a trellis diagram indicating the sequence of the tree codesequence, the number of the dummy bits for the first bit sequence is{max(A, BN/rk!)-A} and the number of the dummy bits for the second bitsequence is {max(A, BN/rk!)-BN/rk}·rk/N. In addition, the predeterminednumber is rk/N, and the modulator 35 modulates the carrier wave signalby 2.sup.(1+n/N) levels.

According to this transmitter shown in FIG. 7, the word length of thetransmission unit is corrected to a value adapted to each desired codinglevel, and the coded modulation is applied with the combination of therepetition coding and the tree coding. For this reason, the transmissioninformation having various formats can be transmitted by carrying outthe coded modulation, and at the receiving end, it is possible to reducethe bit error and to efficiently carry out the maximum likelihooddecoding based on a simple processing procedure.

Next, a description will be given of the operating principle of areceiver according to the second aspect of the present invention, byreferring to FIG. 8.

A receiver includes a demodulator 51, a decoder 52, a demapping part 53and a word length corrector 54 which are connected as shown in FIG. 8.The demodulator 51 obtains a reception sequence by demodulating areception wave received from the transmitter shown in FIG. 7 via theradio channel. The decoder 52 generates a decoded symbol sequence bycarrying out a maximum likelihood decoding with respect to the receptionsequence obtained by the demodulator 51. The demapping part 53 reads thedecoded symbol sequence generated by the decoder 52, and restores thefirst bit sequence and the second bit sequence which are generated bythe divider 31 of the transmitter shown in FIG. 7 by carrying out ademapping adapted to the format of the set partitioning carried out bythe mapping part 34 of the transmitter shown in FIG. 7. The word lengthcorrector 54 reads the first bit sequence and the second bit sequencewhich are obtained by the demapping part 53, and restores thetransmission unit by eliminating the dummy bits which are added to thefirst bit sequence and the second bit sequence by the divider 31 of thetransmitter shown in FIG. 7.

According to this receiver shown in FIG. 8, it is possible to positivelyrestore the transmission information (transmission unit) indicated bythe reception wave which is received from the transmitter via the radiochannel.

Next, a description will be given of a first embodiment of thetransmitter and the receiver according to the present invention. Thisfirst embodiment employs the first aspect of the present inventiondescribed above.

FIG. 9 is a system block diagram showing this first embodiment of thetransmitter and the receiver according to the present invention. In FIG.9, a transmitter 80 and a receiver 90 are located at opposite ends via aradio channel RC.

The transmitter 80 includes a repetition encoder 81, a convolutionalencoder 82, a mapping circuit 83, and an M-ary modulator 165 which areconnected as shown in FIG. 9. The repetition encoder 81 corresponds tothe repetition encoder 11 shown in FIG. 5, and the convolutional encoder82 corresponds to the tree encoder 12 shown in FIG. 5. The mappingcircuit 83 corresponds to the mapping part 13 shown in FIG. 5, and theM-ary modulator 165 corresponds to the modulator 14 shown in FIG. 5.

On the other hand, the receiver 90 includes a branch metric calculator91, an add-compare-select (ACS) part 92, a path metric memory 93, a pathmemory 94, and a demodulator 166 which are connected as shown in FIG. 9.The branch metric calculator 91, the ACS part 92, the path metric memory93 and the path memory 94 correspond to the decoder 42 and the demappingpart 43 shown in FIG. 6. The demodulator 166 corresponds to thedemodulator 41 shown in FIG. 6.

At the transmitter 80, out of the transmission information that isdivided into units of 25-bit words each made up of a bit a₁ and bits b₁through b₂₄, the bit a₁ is input to the repetition encoder 81 while thebits b₁ through b₂₄ are input to the convolutional encoder 82. Outputsof the repetition encoder 81 and the convolutional encoder 82 areconnected to corresponding inputs of the mapping circuit 83. An outputof this mapping circuit 83 is connected to an input of the M-arymodulator 165. An output of the M-ary modulator 165 connects to theradio channel.

In addition, at the receiver 90, the demodulator 166 demodulates atransmitting information received via the radio channel, and inputs areception sequence to the branch metric calculator 91. An output of thisbranch metric calculator 91 is connected to one input of the ACS part92. One output of the ACS part 92 is coupled to the other input of theACS part 92 via the path metric memory 93. The other output of the ACSpart 92 is connected to an input of the path memory 94, and a decodedresult is obtained from an output of this path memory 94.

In the transmitter 80, the repetition encoder 81 carries out arepetition coding process with respect to the bit a₁ at a rate 1/r,where r is a predetermined integer, and successively generates r-bitcode words (hereinafter referred to as repetition code words).

In addition, the convolutional encoder 82 successively carries out aconvolutional coding process which is based on a constraint length K anda rate R (=k/n) with respect to a code block length n, with respect tobits b_(i) through b_(i+r'k-1), where i=1, 2, . . . . The bits b_(i)through b_(i+r'k-1) have a bit length r'k with respect to a real numberr' and an information block length k which are given by a formula r'=r/Nwith respect to the predetermined integer r and a number N of symbols tobe indicated by a single branch of a trellis diagram. Hence, code wordsc_(l) through c_(l+r'n-1) (hereinafter referred to as convolutional codewords) having a word length r'n are generated by the convolutionalencoder 82, where l=1, r'n, . . . .

The mapping circuit 83 generates data amounting to r symbols which canbe transmitted according to an M-ary modulation scheme having 2^(M)signal points with respect to a number M satisfying a formula r+r'n=Mr,by dividing the repetition code words and the convolutional code wordsinto a most significant bit (MSB) and subsequent lower significant bitsbased on the set partitioning. Furthermore, during this process ofgenerating the data, the mapping circuit 83 sets a least squareEuclidean distance d² _(Emin) (l_(i)) to satisfy a formula d² _(Emin)(l_(i))=Δ_(i) ·d_(min) (i) with respect to a distance Δ_(i) between thesignal points and a coding level (minimum distance of codes) d_(min)(i).

The M-ary modulator 165 generates a transmitting wave signal bysubjecting a carrier wave signal to a 8-phase phase shift keying (PSK)modulation scheme depending on the values of the symbols generated bythe mapping circuit 83. Under such a modulation, the distances betweenthe signal points respectively become Δ₁ =0.58 and Δ₂ =2, the value ofthe least square Euclidean distance d² _(Emin) becomes "8", and thevalues of the coding levels d_(min) (1) and d_(min) (2) respectivelybecome "14" and "4".

The above described conditions stand when the repetition coding iscarried out at a rate of "1/14", and the convolutional coding is carriedout at a rate of "3/4" and a constraint length of "4", for example.

Furthermore, when the repetition coding and the convolutional coding arecarried out in this manner, a band efficiency (r'k+1)/r becomesapproximately 1.57 bits/symbol which is approximately 20% smaller than aband efficiency (=2 bits/symbol) in the case of the trellis coding andthe block coding. However, since the value of the least square Euclideandistance d² _(Emin) becomes "8" which is double the value of theconventional trellis coding and block coding, this embodiment canfurther obtain a coding gain of approximately 1.5 dB.

This embodiment does not use a non-coded bit level and carries out amaximum likelihood decoding. For this reason, it is possible to reducethe degradation of the performance particularly over a fading channel,and the performance is further improved as compared to that of theconventional system.

On the other hand, in the receiver 90, information indicating thetrellis diagram of the code sequence given by the transmitting wavesignal is supplied in advance to the branch metric calculator 91. Hence,with respect to the reception sequence obtained from the demodulator166, the branch metric calculator 91 calculates the branch metric of allbranches b_(ij) from each state S_(i) at a time t to a state S_(j) at atime t+1.

With respect to all such branches b_(ij), the ACS part 92 updates thepath metric by carrying out a arithmetic operation indicated by aformula σ_(i),t+1 =σ_(i),t +λ(y_(t), b_(ij)) by carrying out thefollowing operations (1) through (3), where σ denotes the path metric, λdenotes the branch metric, and y_(t) denotes the received block sequenceat a time t.

(1) With respect to the branches b_(ij), add the path metrics σ_(i),t ofsurvivor paths p_(i),t of each state S_(i) at a time t to the branchmetric λ(y_(t), b_(ij));

(2) For every state S_(j) at the time t+1, compare the sums obtained in(1) above with respect to all paths up to the state S_(j), and select acombination of the survivor path p_(i),t and the branch b_(ij) thatresults in a minimum value; and

(3) Obtain the path linking the survivor path p_(i),t and the branchb_(ij) selected in (2) above as a survivor path p_(i),t+1 of the stateS_(j) and update the path metric followed by the above mentionedformula. During the process of carrying out this arithmetic operation,the path memory 94 stores the survivor path of each state, and the pathmetric memory 93 stores the metric which is updated as described aboveand supplies a subject of a similar operation to be carried out by theACS part 92 depending on the following reception sequence.

In other words, a decoded result corresponding to the column of theselected branch is obtained from the output from the path memory 94. Inaddition, the trellis diagram which is used as a reference for theoperation carried out by the branch metric calculator 91 and the ACSpart 92 is given as a combination of 2 identical sub-trellis diagramsindependently corresponding to the logic values of the repetition code,as shown in FIG. 10.

Therefore, according to this first embodiment, the coded modulation ispositively realized by a simple structure utilizing the combination ofthe repetition coding and the convolutional coding. In addition, at thereceiving end, the maximum likelihood decoding (Viterbi decoding) ispositively applied when carrying out the decoding process. Hence, asindicated by circular marks and square marks in FIG. 11, it is possibleto form 2 transmission channels having different BERs.

Next, a description will be given of a second embodiment of thetransmitter and the receiver according to the present invention. Thissecond embodiment employs the first aspect of the present inventiondescribed above.

FIG. 12 is a system block diagram showing this second embodiment of thetransmitter and the receiver according to the present invention. In FIG.12, a transmitter 80A and a receiver 90A are located at opposite endsvia a radio channel RC. In FIG. 12, those parts which are essentiallythe same as those corresponding parts in FIG. 9 are designated by thesame reference numeral with an affix "A", and a description thereof willbe omitted.

The transmitter 80A includes a divider 84, a repetition encoder 81A, aconvolutional encoder 82A, a mapping circuit 83A, and an M-ary modulator165A which are connected as shown in FIG. 12. The divider 84 correspondsto the divider 21 shown in FIG. 5. The repetition encoder 81Acorresponds to the repetition encoder 11 shown in FIG. 5, and theconvolutional encoder 82A corresponds to the tree encoder 12 shown inFIG. 5. The mapping circuit 83A corresponds to the mapping part 13 shownin FIG. 5, and the M-ary modulator 165A corresponds to the modulator 14shown in FIG. 5.

On the other hand, the receiver 90A includes a branch metric calculator91A, an ACS part 92A, a path metric memory 93A, a path memory 94A, ademodulator 166A, and a combiner 95 which are connected as shown in FIG.12. The branch metric calculator 91A, the ACS part 92A, the path metricmemory 93A, the path memory 94A and the combiner 95 correspond to thedecoder 42 and the demapping part 43 shown in FIG. 6. The demodulator166A corresponds to the demodulator 41 shown in FIG. 6.

In the transmitter 80A, the divider 84 successively reads thetransmission information in synchronism with the transmissioninformation. Further, based on the format of the transmissioninformation, such as the frame structure of the transmissioninformation, and a combination of upper limit values of the BER to besecured under this format, the divider 84 divides bits included in theindividual transmission information into 2 groups, and supplies onegroup to the repetition encoder 81A and the other group to theconvolutional encoder 82A. For the sake of convenience, it is assumedthat one group consists solely of a bit a₁ which is supplied to therepetition encoder 81A, and the other group consists of bits b₁ throughb₂₄ which are supplied to the convolutional encoder 82A. The operationsof the repetition encoder 81A, the convolutional encoder 82A, themapping circuit 83A and the M-ary modulator 165A are basically the sameas those of the first embodiment described above.

On the other hand, in the receiver 90A, the The operations of thedemodulator 166A, the branch metric calculator 91A, the ACS part 92A,the path metric memory 93A and the path memory 94A are basically thesame as those of the first embodiment described above.

Therefore, according to this second embodiment, the coded modulation ispositively realized by a simple structure utilizing the combination ofthe repetition coding and the convolutional coding. In addition, at thereceiving end, the maximum likelihood decoding (Viterbi decoding) ispositively applied when carrying out the decoding process. Hence, it ispossible to form 2 transmission channels having different BERs. Thesefeatures can be obtained by adapting to the format of the transmissioninformation and/or the combination of the upper limit values of the BERto be secured under the format, under cooperation of the divider 84 andthe combiner 95, thereby increasing the coding gain while flexiblyadapting to the format of the transmission information.

Next, a description will be given of a third embodiment of thetransmitter and the receiver according to the present invention. Thisthird embodiment employs the second aspect of the present inventiondescribed above.

FIG. 13 is a system block diagram showing this third embodiment of thetransmitter and the receiver according to the present invention. In FIG.13, a transmitter 80B and a receiver 90B are located at opposite endsvia a radio channel RC. In FIG. 13, those parts which are essentiallythe same as those corresponding parts in FIG. 9 are designated by thesame reference numeral with an affix "B", and a description thereof willbe omitted.

The transmitter 80B includes a divider 85, a repetition encoder 81B, aconvolutional encoder 82B, a mapping circuit 83B, and an M-ary modulator165B which are connected as shown in FIG. 13. The divider 85 correspondsto the divider 31 shown in FIG. 7. The repetition encoder 81Bcorresponds to the repetition encoder 31 shown in FIG. 7, and theconvolutional encoder 82B corresponds to the tree encoder 32 shown inFIG. 7. The mapping circuit 83B corresponds to the mapping part 33 shownin FIG. 7, and the M-ary modulator 165B corresponds to the modulator 34shown in FIG. 7.

On the other hand, the receiver 90B includes a branch metric calculator91B, an ACS part 92B, a path metric memory 93B, a path memory 94B, ademodulator 166B, and a combiner 96 which are connected as shown in FIG.13. The branch metric calculator 91B, the ACS part 92B, the path metricmemory 93B, the path memory 94B and the combiner 96 correspond to thedecoder 52, the demapping part 53 and the word length corrector 54 shownin FIG. 8. The demodulator 166B corresponds to the demodulator 51 shownin FIG. 8.

In the transmitter 80B, the transmission information is input to thedivider 85 in units of frames (or cells), where each frame is made up of2 fields having different tolerable upper limit values of BER. Thedivider 85 successively reads the frames of the transmission informationin synchronism with the frames, and carries out a process with respectto each of the frames based on the following procedure.

1! Divide contents of the 2 fields, and store the divided contents asmatrixes α and β;

2! Respectively input the matrixes α and β to the repetition encoder 81Band the convolutional encoder 82B as subjects to be encoded; and

3! For each field obtained by the dividing described in 1! above,successively write (M-A) dummy bits to the end of the matrix α andsuccessively write ((M-B/r'k)·r'k) dummy bits to the end of the matrixβ, with respect to an integer M described by a formula M=max(A, B/r'k!)under a Gaussian symbol " !" depending on a number A of bits to besubjected to the repetition coding in the repetition encoder 81B, anumber B of bits to be subjected to the convolutional coding in theconvolutional encoder 82B, a real number r', and an information blocklength k. The end of the matrix α refers to a region adjacent to aregion which stores the end bit of the matrix α forming the transmissioninformation. Similarly, the end of the matrix β refers to a regionadjacent to a region which stores the end bit of the matrix β formingthe transmission information.

For the sake of convenience, it is assumed that the logic values of theabove dummy bits are all "0".

In addition, when the contents of the matrixes α and β become definiteunder the above described process, the divider 85 refers to thesematrixes α and β in an ascending order and reads 1 bit at a time fromthe matrix α and reads r'k bits at a time from the matrix β. The bitsread from the matrix α are successively input to the repetition encoder81B, and the bits read from the matrix β are successively input to theconvolutional encoder 82B.

The operations of the repetition encoder 81B, the convolutional encoder82B, the mapping circuit 83B and the M-ary modulator 165B are basicallythe same as those of the first embodiment described above.

On the other hand, in the receiver 90B, the combiner 96 successivelyreads the decoded results obtained via the path memory 94B (procedures1! through 3!), and restores the transmission information by carryingout a process complementary to the series of processes carried out bythe divider 85 described above. The operations of the demodulator 166B,the branch metric calculator 91B, the ACS part 92B, the path metricmemory 93B and the path memory 94B are basically the same as those ofthe first embodiment described above.

Therefore, according to this third embodiment, the coded modulation ispositively realized by a simple structure utilizing the combination ofthe repetition coding and the convolutional coding. In addition, at thereceiving end, the maximum likelihood decoding (Viterbi decoding) ispositively applied when carrying out the decoding process. Hence, it ispossible to form 2 transmission channels having different BERs. Thesefeatures can be obtained by flexibly adapting to the arrangement and/orsize of the fields included in the frames forming the transmissioninformation, under cooperation of the divider 85 and the combiner 96,thereby increasing the band efficiency and the coding gain whileflexibly adapting to the format of the transmission information.

In each of the embodiments described above, the convolutional coding iscarried out in parallel to the repetition coding, however, the codingcarried out in parallel to the repetition coding is of course notlimited to the convolutional coding.

In addition, with respect to the convolutional coding and the treecoding, a block coding may be used as a substitute coding scheme if asoft decision is made under a sufficient linear reception at thereceiving end.

Furthermore, although the PSK modulation is used in each of theembodiments described above, the present invention is of course notlimited to the use of the PSK modulation. For example, if a codedmodulation based on a desired signal arrangement is possible, it ispossible to use frequency shift keying (FSK) modulation, amplitude shiftkeying (ASK) modulation or quadrature-amplitude modulation (QAM) schemein the present invention.

Moreover, although the arrangements of signal points are not shown forthe embodiments described above, such signal arrangements can be mappedbased on the set partitioning. In addition, the signal arrangements arenot limited to a particular arrangement as long as the degradation ofthe transmission efficiency caused by the increase of the peak power ofthe transmitting wave can be suppressed to a tolerable range.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A transmitter comprising:a repetition encodergenerating a repetition code sequence having a word length of r bits bycarrying out a repetition coding with respect to 1 bit of a transmissionunit which divides transmission information by a predetermined wordlength, where a rate of the repetition coding is set equal to areciprocal of a predetermined integer r; a tree encoder generating atree code sequence by carrying out a tree coding with respect to theother bits of the transmission unit, where a rate of the tree coding isset to k/n, k denotes an information block length and n denotes a codeblock length; a mapping part carrying out a mapping with respect to acombination of the repetition code sequence generated by said repetitionencoder and the tree code sequence generated by said tree encoder, basedon a set partitioning while maintaining correspondence between themapping and the transmission unit; and a modulator generating atransmitting wave signal by carrying out an M-ary modulation scehemewith respect to a carrier wave signal based on the combination which ismapped by said mapping part, and transmitting the transmitting wavesignal to a radio channel, said predetermined word length being (1+rk/N)bits, where N denotes a number of transmission symbols indicated by asingle branch of a trellis diagram indicating a sequence of the treecode sequence, said modulator modulating the carrier wave signal by2.sup.(1+n/N) levels.
 2. The transmitter as claimed in claim 1, whereinthe transmission unit is given by a format which is made up of acombination of 1 bit corresponding to an order of tolerable upper limitvalues of a bit error rate and the other bits, said transmitter furthercomprising:a divider dividing the transmission unit into the 1 bit andthe other bits based on the format, and supplying the 1 bit to saidrepetition encoder the other bits to said tree encoder.
 3. Thetransmitter as claimed in claim 1, wherein said modulator carries out anM-ary modulation sceheme selected from a group consisting of phase shiftkeying modulation, frequency shift keying modulation, amplitude shiftkeying modulation and quadrature-amplitude modulation.
 4. Thetransmitter as claimed in claim 1, wherein said tree encoder carries outa block coding.
 5. A transmitter comprising:a divider readingtransmission units which are given by a format in which transmissioninformation is divided by a word having a bit length A+B which is equalto a sum of A bits corresponding to an order of tolerable upper limitvalues of a bit error rate and B bits, and generating a first bitsequence and a second bit sequence by isolating the A bits and the Bbits from the transmission unit based on the format and adding firstdummy bits and second dummy bits to the A bits and the B bits,respectively; a repetition encoder generating a repetition code sequenceby extracting 1 bit at a time from the first bit sequence generated bysaid divider and carrying out a repetition coding at a rate equal to areciprocal of a predetermined integer r; a tree encoder generating atree code sequence by extracting a predetermined number of bits at atime from the second bit sequence generated by said divider and carryingout a tree coding at a rate k/n, where k denotes an information blocklength and n denotes a code block length; a mapping part carrying out amapping with respect to a combination of the repetition code sequencegenerated by said repetition encoder and the tree code sequencegenerated by said tree encoder, based on a set partitioning whilemaintaining correspondence between the mapping and the transmissionunit; and a modulator generating a transmitting wave signal by carryingout an M-ary modulation scheme with respect to a carrier wave signalbased on the combination which is mapped by said mapping part, andtransmitting the transmitting wave signal to a radio channel, withrespect to a number N of transmission symbols indicated by a singlebranch of a trellis diagram indicating a sequence of the tree codesequence, a number of the first dummy bits being {max(A, BN/rk!)-A} anda number of the second dummy bits being {max(A, BN/rk!)-BN/rk}·rk/N,said predetermined number being rk/N, said modulator modulating thecarrier wave signal by 2.sup.(1+n/N) levels.
 6. The transmitter asclaimed in claim 5, wherein said modulator carries out an M-arymodulation scheme selected from a group consisting of phase shift keyingmodulation, frequency shift keying modulation, amplitude shift keyingmodulation and quadrature-amplitude modulation.
 7. The transmitter asclaimed in claim 5, wherein said tree encoder carries out a blockcoding.